Sea water is almost guaranteed to have more solvents in it than water in a controlled environment like a fish tank. A fish tank would be set up specifically to provide the ideal environment for the fish, while the ocean is naturally occurring and has no such regulation. As far as I can see, it makes perfect sense for water from a fish tank to be less dense than water in the ocean.
There is nothing in the movie to support the contention that tank water is materially different from sea water: Nemo doesn't appear to react any differently. There is no evidence in the real world either. Nemo's friends aren't freshwater fish, and even then I very much doubt a bag of fresh water would float that much. I remain surprised that this is even debated.
Here's a website
documenting that even the most dramatic difference, between salt and freshwater, is only 2.7% density. Hardly enough to account for what we see in that scene. Here's a site specifying tank density of 1.022-1.024. Wikipedia lists the average density of seawater as 1.025. And seawater on the coast would be slightly less dense as it mixes with fresh water runoff—if anything the bags might be slightly negatively buoyant, and sink.
Listed as goof at moviemistakes.com.
I must admit I'm a little bit puzzled by the insistence that water can float on top of water. The initial note added under Hollywood Density was incorrect, the "remove don't respond" edit was incorrect—should have removed just the note—and the assertion that fish tank water is less dense than seawater is highly questionable. IM Db is hardly an authoritative source but this is listed as a "goof" on the movie's entry.
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